1. Technical Field
The embodiments described herein generally relate to methods, tests, and devices for analyzing the concentration and crystalline lattice location of amounts of nitrogen in semiconductor materials.
2. Description of the Related Art
Generally, the military's and more specifically, the Army's overwhelming need for enhanced night-vision monitoring/detection tools is well known within the military community as well as those skilled in the art. Consequently, in order to efficiently meet this need, infrared (IR) detector materials having high detectivity and low manufacturing costs associated therewith are desirable. Currently, Mercury Cadmium Telluride (MCT) or HgCdTe (also referred to cadmium mercury telluride) is the material technology of choice utilized for its adaptability and high performance. In particular, MCT is an alloy made of Cadmium Telluride (CdTe) and Mercury Telluride (HgTe) wherein the amount of Cd in the alloy composition can be chosen so as to tweak or tune the optical absorption of the material to the desired wavelength. Thus, MCT is adaptable to achieve an end-user specified desired optical absorption. Further, MCT is known as being the only material having the ability to detect IR radiation in both of the accessible atmospheric windows, i.e., the mid-wave infrared window (MWIR), which ranges from 3 to 5 μm and the long-wave window (LWIR), which ranges from 10-12 μm. However, MCT is a Group II-VI material that has limited use with regard to other applications and, furthermore, can only be processed in dedicated fabrication facilities. As such, the use of MCT can be very costly.
In contrast, Group III-V materials have many commercial and military applications. Thus, an IR material technology that utilizes and builds upon the existing industrial III-V infrastructure could realize significant cost savings.
In particular, it has been determined that by combining nitrogen with the group V element(s) in a III-V semiconductor with group III elements like Ga, In and Al, materials with relatively large bandgaps can be formed. In recent years it has been realized that addition of small amounts (<10%) of nitrogen to other III-V compounds has the effect of strongly reducing the bandgap. This effect has been used to develop materials in which bandgaps can be tailored and yet are still lattice-matched to available substrates. These compounds are commonly called “dilute-nitrides”—referring to the relatively small concentration of nitrogen. Specifically, the effect has been used to develop materials with bandgaps that allow light absorption and emission at 1.6 micron—the wavelength of maximum transmission of fiber optic cables. These materials use alloys of (Al,Ga,In) (As,Sb,N) which can be lattice matched to GaAs. More recently it has been proposed that the bandgap-narrowing effect can be used in materials with even smaller bandgaps, targeting detectors for the long and very-long infrared range. (These materials would typically use GaSb-, InAs-, or InSb-substrates.) These materials would provide a lower-cost, high-performance, III-V-based alternative to current II-VI materials. Dilute nitrides are therefore expected to have a significant technological impact on both the commercial and military electronic and electro-optics markets.
Materials synthesis techniques for dilute-nitrides are in their infancy and much work remains to be done to optimize the materials quality. One significant issue is control of the incorporation of nitrogen such that all of the nitrogen enters the crystal at substitutional group V sites. There is some evidence that a significant amount of the incorporated nitrogen resides in interstitial sites, forming unwanted electrically and optically-active centers. Some of this evidence comes from ion beam analysis experiments using resonant nuclear reaction analysis (RNRA). For example the reaction:R1=3He+14N→16O+protonhas been used. Unfortunately, the probability of ion-atom interaction, i.e., the reaction cross section for this reaction is extremely small, making measurements time consuming. Additionally, this method requires the use of a 3He ion beam, which can contaminate beamline components and lead to unwanted background reactions in future experiments.
To overcome these shortcomings the state of the art, molecular beam epitaxy (MBE) systems and methods provide for, among other things, the combination of a novel way of synthesizing dilute-nitrides together with an optimum RNRA reaction method that will maximize the sensitivity of the analysis method. It has been realized that the sensitivity improvement will be more than two orders of magnitude over previously used methods.
In the certain desirable embodiments described herein, dilute-nitride films are grown using MBE. The films may be grown on any available substrate and can target any desired wavelength. In the common MBE process, nitrogen is introduced to the growth reactor as a gas and can be excited in a plasma source to produce chemically reactive nitrogen that can be incorporated in the film. Previously all such work has used common nitrogen gas, which is a mixture of 14N and the isotope 15N. The natural abundance of 15N is 0.366% with the remaining 99.96% being 14N. Thus, a reaction targeting 15N as an indicator of the total amount of nitrogen would have limited sensitivity. 15N has been used with success for analysis of nitrogen incorporation at the percent level in gun barrel steel. Conversely, in the embodiments disclosed herein, the films are grown using a supply of nitrogen gas enriched with the 15N isotope. 15N is available in concentrations approaching 100% thus giving rise to a sensitivity enhancement of a factor of 272 (=99.96/0.366) compared to the case when nitrogen with the natural abundance is used. An additional advantage with the method is that semiconductors, grown almost entirely with 15N, and that are exposed to protons of a specific energy that cause the selected nuclear reaction to occur, such material can be p-doped by the carbon atoms created by the process.